Optical fiber systems are known to be sensitive to a variety of different sources of “optical noise” (extraneous signals at wavelengths other than the desired wavelength(s)) that result in impairing the system performance. Various types of filtering arrangements have been proposed through the years to address this problem. Discrete filtering elements (incorporating multiple thin film layers) have been used to remove selected wavelengths from propagating along the fiber. While such discrete filters may be able to reduce the accumulated noise power, they may not sufficiently reduce other system impairments, such as power lost due to noise amplification. Going forward, a distributed, in-line fiber filter is considered to be a more desirable solution than the use of discrete devices, especially for amplifier applications where splice losses, power lost to noise amplification, etc. can effect overall amplifier performance.
Bragg gratings may be formed within the core region of the transmission fiber as one such type of in-line fiber filter to “reflect” selected wavelengths and prevent further propagation of undesirable signal components. See, for example, U.S. Pat. No. 5,717,799 issued to A. Robinson on Feb. 10, 1998, describing the use of a Bragg grating, where the grating is particularly configured to be chirped and apodized to improve the filter qualities. While various in-line arrangements have been successful in providing some filtering, reflection gratings are problematic for in-line filtering in amplifiers, since reflections can lead to unwanted oscillations or lasing at the noise wavelengths. Reflecting gratings can be used as discrete filters, but then do not give the advantages of distributed filtering that are important to the present invention. U.S. Pat. No. 6,141,142 issued to R. P. Espindola et al. on Oct. 31, 2000 discusses an example of a fiber amplifier employing distributed filtering. In this case, filtering is provided by tilted (“blazed”) gratings, instead of reflection Bragg gratings. While some distributed filter embodiments can provide effective filtering, this method requires additional processing steps in fiber fabrication, and restricts the dopant profile of the fiber to those with appropriate photosensitivity.
Thus, a need remains in the prior art for an arrangement that provides optical filtering with enhanced wavelength selectivity, preferably using an in-line, distributed, all-fiber arrangement that eliminates the need to include discrete devices in the optical communication system.